An accumulator or single tube hydraulic damper has, for example, a free piston which is enclosed in a cylinder or housing so as to be free to slide in order to separate a liquid chamber and a gas chamber.
In an accumulator, temperature variation of the liquid volume in a liquid system is compensated by axial displacement of the free piston in the cylinder so as to increase the capacity of the liquid chamber while decreasing the capacity of the gas chamber, or vice versa.
In a hydraulic damper, variation in the capacity of the liquid chamber due to elongation and contraction of the piston rod is also compensated by the axial displacement of the free piston in the cylinder.
The free piston separating the liquid chamber and gas chamber, is generally provided with a seal member such as an O-ring on its outer circumference. The seal member is arranged to be in contact with the inner circumference of the cylinder under a predetermined contact pressure.
When the free piston axially slides in the cylinder, friction occurs between the seal member and the inner circumference of the cylinder, and the free piston does not start to slide until the differential pressure between the liquid chamber and the gas chamber surpasses the frictional resistance between the seal member and the inner circumference of the cylinder. This operation characteristic of a free piston may bring about an unfavorable effect on the response and reliability of hydraulic equipment that has a built-in free piston. In a hydraulic damper, specifically, this characteristic of a free piston may render the damping characteristic unstable.